Sulphur continues to be the most commonly used vulcanizing agent for unsaturated rubbers, e.g. natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR) and styrene-butadiene rubber (SBR). From about 0.25 to 5.0 parts by weight of sulphur, based on 100 parts by weight of rubber, are used in the production of soft rubber. The amount of sulphur effectively used depends on the selected amount of vulcanization accelerator, and this is ultimately determined via the vulcanizate properties desired.
Vulcanization systems very frequently used are the conventional vulcanization system and the semi-efficient vulcanization system. The conventional vulcanization system has high sulphur content and low content of vulcanization accelerator, whereas the semi-efficient vulcanization system has a moderate proportion of sulphur and of vulcanization accelerator. The typical proportions are known to the person skilled in the art. By way of example, they are described in W. Hofmann, Kautschuk-Technologie [Rubber Technology], Genter Verlag, Stuttgart, 1980, p. 64 and 254-255. Conventional vulcanization systems give vulcanizates with good resistance to dynamic load, but these are very susceptible to ageing and reversion. Semi-efficient vulcanization systems usually give vulcanizates which have less resistance to dynamic load but are somewhat more resistant to ageing and reversion.
Reversion is a network-bridging-rearrangement process which takes place on heating in the absence of oxygen, causing impairment of service properties of the vulcanizate and therefore being undesirable (anaerobic ageing). Reversion inevitably takes place in one instance during the vulcanization of very thick and voluminous components, e.g. truck tyres and fenders. Physical explanation of this is simple: when the inner parts of the rubber mixture have been vulcanized to exactly the right extent via the heat flux introduced by way of the hot vulcanization mould, those parts of the rubber mixture that are immediately adjacent to the hot vulcanization mould have naturally by this stage been over-vulcanized. Secondly, heat build-up occurs in the rubber component during its use when there is prolonged, intensive dynamic load due to hysteresis deficiencies (in tyre flexing), the result of this being vulcanizate reversion. The extent of reversion can even be sufficient to cause breakdown of the vulcanizate.
In recent years, some specialized reversion stabilizers have been disclosed, and these either minimize reversion via incorporation of network bridges which are thermally stable and practically incapable of reversion (cf. EP 530 590), or replace the fractured conventional sites in the network by other more stable sites after reversion has occurred (cf. R. N. Datta and W. F. Helt, Rubber World, August 1997, p. 24 seq).
Examples of specialized reversion stabilizers available commercially are the disodium salt of hexamethylene 1,6-dithiosulphate dihydrate and 1,3-bis(citraconimidomethyl)benzene.
A general disadvantage of these commercially available specialized reversion stabilizers is their relatively high price, partly the result of starting materials available only in limited quantities, and also of the difficult and complicated preparation of these products, therefore preventing any broad use in the rubber-processing industry, which is subject to constant pressure for cost reduction, and especially in the tyre industry. One specific disadvantage of the disodium salt of hexamethylene 1,6-dithiosulphate dihydrate is its inconvenient supply form. Because it has the character of a salt, it has to be very finely ground to permit good incorporation by mixing, but the result of this is that the powder has to be oil-coated for reasons of health and safety at work, to suppress dusting.
One specific disadvantage of 1,3-bis(citraconimidomethyl)benzene is that within the vulcanizate it can become active only when reversion has already begun in the sulphur-crosslinked unsaturated rubber, and when, therefore, conjugated olefins have formed, which themselves can undergo a post-crosslinking reaction with the citraconic derivative (via a Diels-Alder reaction) to give a new network, but a network of a different type.
A disadvantage of the vulcanizing agents of EP 530 590 is that their molecular weight is high when compared with the actual species active in crosslinking.
The preparation of α,ω-alkanedithiols derivatized by zinc, by cadmium, by indium, by thallium, by copper or by silver has long been known. For example, H. E. Mabrouk et al. (Inorganica Chimica Acta, 145 (1988), 237-241) describes the direct electrochemical synthesis from zinc or cadmium to give the corresponding derivatives. There is no description of use of a crosslinking agent for unsaturated diene rubbers.
EP 0 383 150 describes the use of cobalt hexanyidithiol or of nickel hexanyldithiol as an adhesion promoter for adhesive mixtures based on unsaturated rubbers. These mixtures are usually crosslinked by adding large amounts of sulphur, and the adhesion promoter is intended to improve adhesion between the sulphur-crosslinked rubber matrix and the reinforcing material (brass-plated steel cord). There is no description of any use as crosslinking agent.
Wide-scale production of vulcanizates from unsaturated rubbers usually uses only sulphur and accelerators as vulcanizing agents, i.e. uses no agents which prevent or reduce reversion. However, rubber vulcanizates produced conventionally, using conventional and semi-efficient vulcanization systems, have unsatisfactory properties. There is therefore a need for a vulcanizing agent for unsaturated rubbers which is largely based on components which are readily available in large quantities at low cost, can partially or completely replace exudation-susceptible crystalline sulphur, and which gives vulcanizates with improved resistance to reversion, in particular after overheating.